AQA AS Level Biology Paper 2, June 2024: Question 3

9 marks · Medium difficulty · Extended Answer

Describe the process of cell fractionation and ultracentrifugation to isolate mitochondria, and evaluate data on whether resistance exercise training improves athletic performance.

Practise this question

Question

Question 03.1 asks to describe how to use cell fractionation and ultracentrifugation to obtain a sample of mitochondria from muscle tissue (4 marks). Question 03.2 provides context on a study with 11 young males undergoing a 12-week resistance exercise training (RET) programme. A bar graph (Figure 3) plots mitochondrial respiratory capacity per mg of muscle tissue in arbitrary units against 'Before RET' (approximately 54 units) and 'After RET' (approximately 74 units). Error bars representing plus or minus 2 standard deviations are shown with no overlap. Students are asked to evaluate whether 12 weeks of RET would improve athletic performance in the general population (5 marks).
Question text

03.1 Describe how you would use cell fractionation and ultracentrifugation to obtain a

sample of mitochondria from muscle tissue.

[4 marks]

03.2 Mitochondrial respiratory capacity is a measure of maximum ATP production in a

mitochondrion. Scientists investigated the effect of a resistance exercise training

(RET) programme on the respiratory capacity of mitochondria in skeletal muscle

tissue. RET develops muscle strength.

The scientists:

• took samples of muscle tissue from 11 young males before and after a 12-week

RET programme

• recorded the respiratory capacity of the mitochondria in the samples of muscle

tissue.

Figure 3 shows some of the scientists’ results.

The error bars represent ± 2 standard deviations from the mean, which includes over

95% of the data.

Figure 3

Using all the information, evaluate whether 12 weeks of RET would improve athletic

performance in the general population.

[5 marks]

Mark scheme

Show the mark scheme Mark scheme for 03.1 gives 4 marks: 1. Cold, buffered solution with same water potential (isotonic); 2. Homogenise tissue and filter; 3. Centrifuge at low speed and remove supernatant or separate nuclei; 4. Centrifuge supernatant at higher speed to pellet mitochondria. Mark scheme for 03.2 gives up to 5 marks with MP1 compulsory: (For) 1. SDs do not overlap showing significant increase; 2. Increase in ATP linked to muscle contraction or exercise capacity; (Against) 3. Only males tested; 4. Small sample size of only 11; 5. RET may not strengthen required muscles or athletic performance not directly measured; 6. Only young individuals tested; 7. Unknown prior exercise levels.

Question Marking Guidance Mark Comments

1. Accept isotonic for

1. Use cold, buffered solution with same water same water potential.

potential; 2. Accept blend OR

2. Homogenise tissue and (then) filter; grind for homogenise.

3. Centrifuge at low speed and remove supernatant 3. and 4. Accept

03.1 (4 x solution for

OR AO1)

supernatant.

Centrifuge at low speed to remove/separate nuclei;

3. Accept

4. Centrifuge supernatant at higher speed (and heavier/denser

pellet containing mitochondria is mixed with new organelles

supernatant);

MP1 must be included for max marks. 1. Accept

(For) ‘difference/increase is

not due to chance’ for

1. SDs do not overlap so significant significant

difference/increase (in respiratory capacity); difference/increase.

2. Increase in ATP linked to increase in muscle

contraction; 1. Accept ‘error bars’

for SDs

OR

Increase in ATP linked to increase in exercise

capacity;

(Against)

3. Only males (in investigation)

OR 5 max

03.2 No females (in investigation); (5 x

4. Only 11 (individuals in investigation) AO3)

5. Accept examples of

OR athletic performance

Small sample size; eg running.

5. RET may not strengthen required muscles Ignore reference to

‘no statistical test’

OR

Don’t know which (skeletal) muscles affected

OR

Athletic performance not investigated;

6. Results may differ depending on age

OR

– – –

Only young (individuals) tested;

8 7. No idea of exercise done by individuals before

investigation;

How to answer it

Cell Fractionation & Resistance Training Evaluation

📋 What this question tests
  • Cell biology practical skills: Standard 4-step protocol for cell fractionation and differential centrifugation (ice-cold, isotonic, buffered medium, homogenisation, filtering, and centrifugation speeds).
  • Organelle sedimentation order: Knowledge that heavier organelles (nuclei) sediment at lower speeds than lighter ones (mitochondria).
  • Data evaluation & scientific method: Interpreting standard deviation (SD) error bars, linking ATP production to physiological function, and identifying critical study limitations (sample size, sex bias, age bias, unmeasured variables).
Part (a) — Question 03.1 [4 Marks]

Isolating Mitochondria via Cell Fractionation

AO1: Knowledge of Practical Procedures

✅ Model Answer (Full 4 Marks)

  1. Preparation medium: Place muscle tissue in a cold, isotonic (same water potential), and buffered solution.
  2. Homogenisation & filtration: Homogenise (blend/grind) the tissue in a homogeniser to break open cells, then filter the homogenate to remove unbroken cells and large debris.
  3. Low-speed spin: Centrifuge the filtrate at a low speed; nuclei (heaviest organelles) form a pellet at the bottom. Separate and keep the liquid supernatant.
  4. High-speed spin: Centrifuge the supernatant at a higher speed; mitochondria sediment out to form a pellet at the bottom.

💡 Why the Three Solution Conditions Matter

  • Ice-cold: Reduces kinetic energy to slow down/stop autolytic enzyme activity that could break down organelles.
  • Isotonic (same water potential): Prevents osmotic movement of water into or out of organelles, stopping them from bursting (lysis) or shrinking (crenation).
  • Buffered: Maintains a constant pH so organelle proteins and membrane-bound enzymes do not denature.

🧠 Exam Technique: Organelle Order

Remember the sedimentation order from heaviest to lightest:

Nuclei → Chloroplasts → Mitochondria → Lysosomes → Endoplasmic Reticulum / Ribosomes

Mnemonic: Naughty Clever Monkeys Like Eating Raspberries.

Since muscle cells lack chloroplasts, nuclei pellet first at low speed. Mitochondria require the next, higher speed spin.

❌ Common Student Mistakes

  • Forgetting filtration: Students often jump straight from homogenisation to spinning. Filtration removes whole cells and connective tissue.
  • Spinning at high speed first: Spinning too fast immediately forces all organelles to pellet together, failing to isolate mitochondria.
  • Confusing pellet and supernatant: Throwing away the supernatant after the first spin loses all the mitochondria.
Mark Breakdown: 1 mark for cold, buffered, isotonic solution • 1 mark for homogenise and filter • 1 mark for centrifuging at low speed to pellet nuclei / collecting supernatant • 1 mark for centrifuging supernatant at higher speed to pellet mitochondria.
Part (b) — Question 03.2 [5 Marks]

Evaluating Resistance Exercise Training (RET)

AO3: Critical Data Evaluation & Drawing Valid Conclusions

📐 Data Reading & Error Bar Analysis

  • Before RET: Mean = 54 arbitrary units; error bars span ~48 to 60 units.
  • After RET: Mean = 75 arbitrary units; error bars span ~68 to 81 units.
  • Gap between bars: The upper limit before RET (60) is distinctly lower than the lower limit after RET (68).
  • Conclusion: The ±2 SD bars do not overlap, demonstrating a statistically significant increase that is unlikely to be due to chance.

✅ Model Answer (5 Marks)

Evidence FOR:

  • Standard deviations do not overlap: Therefore, there is a significant increase in mitochondrial respiratory capacity (the difference is not due to chance). [Compulsory Marking Point!]
  • Physiological link: Increased respiratory capacity provides more ATP, which is required for muscle contraction and increased exercise/athletic endurance.

Evidence AGAINST / Limitations:

  • Sex bias: Tested only on males (no females), so findings cannot be generalised to women.
  • Small sample size: Only 11 individuals tested, which reduces statistical reliability.
  • Age bias: Only young individuals tested; older demographics may respond differently.
  • Task specificity: RET tests muscle strength/isolated tissue, but overall "athletic performance" (e.g. running, stamina) was never directly measured.

🧠 Exam Strategy: "Evaluate" Questions

  • Balance is mandatory: Always provide points supporting the claim and points opposing/limiting the claim.
  • Check compulsory points: The mark scheme states "MP1 must be included for max marks". Whenever standard deviation bars are shown, you must comment on whether they overlap and what that means for statistical significance.
  • Scrutinise the prompt: The question asks about the "general population". Immediately check: age, sex, sample size, health status, and prior training history.

❌ Pitfalls to Avoid

  • Claiming RET "proves" improved athletic performance: The scientists measured isolated mitochondrial capacity per mg of tissue, not race times, VO₂ max, or real-world athletic performance.
  • Saying "no statistical test was done": The mark scheme explicitly states: Ignore reference to 'no statistical test' because non-overlapping ±2 SD bars (representing ~95% confidence intervals) already demonstrate significance.
  • Vague claims: Stating "small sample" without citing the exact number given in the stem (only 11 men).
Mark Breakdown: Max 5 marks. Must include MP1 (non-overlapping SDs indicate significant increase). Remaining 4 marks drawn from biological link (ATP → muscle contraction) and at least three valid study limitations (male-only, 11 participants, young-only, athletic performance not directly measured, prior exercise levels unknown).

Topics

Biology · Practical skills · 3.2 Cells · 3.1 Biological molecules · Experimental design · Data analysis · Uncertainty and evaluation

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.